(1) Field of the Invention
The present invention relates to an exhaust purification system that is suited to purify engine exhaust, particularly diesel engine exhaust.
(2) Description of the Related Art
Heretofore, as a postprocessing system for a diesel engine which is installed in a vehicle, there has been developed an exhaust purification system that includes a diesel particulate filter (DPF) (hereinafter referred to simply as a filter) for collecting particulate matter (hereinafter referred to as PM) contained in exhaust gas, and an oxidation catalyst for oxidizing and removing the PM on the filter. This exhaust purification system generates nitrogen dioxide (NO2) from nitrogen oxide (NO) contained in exhaust gas using the oxidation catalyst, and also continuously burns and removes PM by reacting NO2 with PM collected on the filter, thereby regenerating the filter. Note that such a type for burning and removing PM continuously is called a continuous regeneration type (e.g., see Japanese patent laid-open publication No. 2003-13730).
On the other hand, under the Japanese driving condition in which there are many traffic jams (e.g., vehicles are traveling with engines revolving at low load and low speed), the exhaust gas temperature is less liable to rise, so the temperature of the filter is less liable to reach a temperature required for combustion of PM. That is, there is a possibility that the PM on the filter will not be sufficiently burned and removed.
For this reason, there has also been developed an exhaust purification system of forced regeneration type which performs combustion of PM forcibly by maintaining the filter temperature at the required temperature intentionally. As the forced regeneration type exhaust purification system, many kinds of system have been developed to increase a filter temperature by providing an external heat source such as a heater in the filter or by raising an exhaust gas temperature.
In addition, there has been developed a system of supplying unburned fuel (HC) to an oxidation catalyst provided in the stage before the filter (upstream of the exhaust passage), and raising the temperature of the filter by heat produced by an oxidative reaction. Since heat generated by unburned fuel can be utilized directly to the temperature rise of the filter, this system can obtain satisfactory thermal efficiency and is expected as a system capable of suppressing the worsening of fuel consumption.
However, in the case of the system which gives heat to the filter from the upstream side of the filter to raise the filter temperature, it is conceivable that because of the radiation of heat from the filter to outside the exhaust passage, a rise in temperature will become slow on the downstream side of the filter. For example, as shown in FIG. 7, in the case where the inlet temperature of the filter is controlled so that it becomes the target temperature required to burn PM, there is a possibility that the outlet temperature of the filter will not reach the target temperature, and consequently, there is a fear of the PM within the filter not being sufficiently burned.
Particularly, since exhaust gas is slight in quantity during idle where the engine is running at relatively low revolutions, heat quantity radiated from the filter to the outside air increases compared with heat quantity of oxidation given by unburned fuel, so there is a large difference in temperature between the inlet side and outlet side of the filter. Therefore, under the driving condition in which exhaust gas is slight in quantity, there is a problem that a part difficult to regenerate may occur within the filter, particularly the outlet side.
In addition, because the heat capacity of the filter is generally large, a difference in temperature rise speed occurs between the inlet side and outlet side of the filter disposed in the exhaust line. For that reason, even if feedback control of the outlet temperature of the filter is performed, satisfactory response is difficult to obtain, and there is another problem that because of an excess rise in the inlet temperature of the filter, melt damage will occur easily.